Method of and means for eliminating distortion



' Nw. M, w39. A. MME@ www@ METHOD OF AND MEANS FOR ELIMINATING DISTONRTION -Filed March 2e, 1957 5 sheets-sheet 1 Nov. 14, 1939. l A. MILLER v S@ y METHOD OF AND MEANS FOR ELIMINATING DISTORTION Filed March .26,- 1937'. 5 Sheets-Sheet .2

j' .3.v 'gyfude Nov.ll4, 1939.r A. MILLER i 2,180,160

METHOD 0F AND MEANS FOR ELIMINATING DISTORTION Filed March 26, 1957. 5 sheets-sheet s A. MILLER Nov. 14, 193?,

METHOD OF ANDMEANS FOR ELIMINATING DISTORTION Filed March 26, 1957 5 sheets-sheet 4 A. MILLER Nov. 14, 1939D METHOD 0F AND-MEANS FOR ELIMNATING DISTORTION Filed March 26, 1937 5 SheetS-Sheell 5 .Q k E., i Ww M M ma NN Q\ Nm, m, 1 %N QN N\ XN. Q W L @L MT v rN. T l nw N N INI mW Iww NAVI. um. Nm .ww w\/ QN Q s@ w w wm mw ww w #Ww MM Patented Nov. 1939 UNITED STATES,

PATENT, Lorsica METHOD 0F AND MEANS roasummu- -nve nrsronrxon mum- Mmer, chaises., man, mmm a sim` born Company, Cambridge, Maw., a corporation of Massachusetts Application March 26, 11837, Serial N120. 133,12

limiting .im wfi'll) The present invention lrelates to' methods of and means for eliminating wave-form distortion, and more particularly such distortion as is introduced into an electric circuit by the presence of series coupling condensers. Though the invention has vgeneral application, itvwili be described herein in connection with the distortion introduced into an electrocardiograph system ofthe amplier type. l

Electrocardicgraphs of the above-described character are commonly provided with electrodes that are positioned over various parts of a human body and through the medium of which the electrical impulses associated with cardiac activity are impressed 'upon the input vof the amplier and,.

v after amplication, are transmitted to an oscillograph. With the aid of the oscillograph, a record may be produced corresponding to the amplied electric currents or potentials.

w From a more specic aspect, therefore, this invention relates to therecording of electric currents or potentials by oscillographic means, and

,particularly to the recording of potentials that are associated with the action of the heart.

Electrocardiographsoi the above-mentioned ampliiier type usually embody several resistancecapacitance-coupled amplier stages coupled by\ series condensers. An additional series condenser of an, electrocardiograph circuit embodying the present invention; Fig. 2 is a similar view 'cfa modification; Figs.` 3 to 7, inclusive, are explanavtory diagrams; Fig. 8 is an actual cardiogram produced in accordance with present-day electrocardiographs oi' the above-described character; Fig. 9 is acorresponding view oi! a cardiogram produced in accordance with the present invention: Fig. 101s a decayl curve illustrating the eii'ect produced by a suddenly introduced constant e voltage; Figs. 11 to '1A are explanatory diagrams;

andlllgs. 15and16areviewssimilartol'lgs.

ranno: modifications. .Electrocardiographs are usually provided with 'three electrodes, twol oi-which are shown -at 2 .Y

and ,thatareconnecteiiu pairato partsctthelmmanbody. Theelectrodesareconthe input circuit of a vacmmi-tuhe am= pliiier B; between the illament and the grid {it} in parallel with a resistance i2. A series conm denser I4 may be inserted between the electrode d and the grid Ill to iilter out the direct-current' g skin effects. lThe output circuit of the tube t is shown extending from the illament t, through a plate battery I6 and a resistance ld, to the plate 2li. Thetube 8 is shown coupled to another tube 22 of the same character as the tube t, in the. l@

input circuit oi' which isdisposed a resistance 2t. The resistance's i8 and 24 are separated by a series coupling condenser 26, as is customary with resistance capacity coupled ampliers. The

output circuit of the second amplier tube 22 is is provided with a plate battery 23 and a resistance 28, and the input'pircuit ci the next-following tube 20 is provided with a resistance 32, the resistances 28 and 32 being similarly separated by a similar series condenser Bt. As'many stages 2@ of amplication as desired may be coupled in this manner, the output circuit oi the last, or

output, tube 3U ofthe amplifier being shown extending from the iilament 86 throughtthe plate battery Il, a coil 40 i'or actuating a galvanometer 25 Il, and a resistance 42, to the plate St.

The resistance 42 is assumed to represent all the resistance that may be present in the output circuit of the tube 30, including the galvanometer coil resistance and any external resistance added 3@ thereto. It will be understood that the diagramv .matic showing is greatly simplified, for clearnesa For example, each of the tubes t, 22 and t@ is shown as a screen-grid tube. thoughthe corrnections for the screen grids are omitted. The 35 filament-heating batteries are not shown for a similar reason; these laments may be connected to a single battery or other source of power, and

the plate batteries I8, 23 and 38 may likewise be combined into a single source of power.

The above-.described distortion is introduced by each, of the condensers I4, 2l and td. The cardiac potentials applied to the terminals 2. and l consister a succession ot impulses. To.

interpret the record made by the galvanometer 45 Il oi' these impulses, it is necessary to know both n the location oi! the base line about which' the potential iluctuates and the amplitude of theA impulses. The base lineis indicated in Mss. 3

toSastlieaxisorabscissaetorepresenttlme.

and amplitudes are shown measured alongthe ordinate. i

n:in Mg. 3,-there is illustrated a theoretically idealtrlanwlarsignalimpuheltsimilartothe on the other hand, the corresponding recorded signal Il is shown considerably distorted. The

theoretically correct amplitude is shown at l0.

The recorded amplitude is shown at I2, somewhat smaller than the amplitude shown at IB. The effect of the series condensers il, 2l and 3l in the recording circuits is, not only to reduce the amplitude of the recorded wave, but also to pro-v ous nature, as it may, at times,

duce a deflection, as indicated at $4, below the original base line 56 after the passage oi the impulse. This deflection is sometimes of a seribe misconstrued as an additional impulse produced by the heart. Such distortion effects have been an unavoidable concomitant of circuits using capacitance couyat 62. at time tz, after a short time interval.

During that time interval between t1 and tz. the

theoretical impulse has a constant amplitude. The corresponding recorded impulse, shown in Fig. 6, however, though initially of the same amplitude as the theoretical impulse, at the initial time t1, shows a decaying amplitude, with a total drop, at the time t2, having a value marked 6. As a result, when the impulse becomes zero, therefore, the record shows an excursion below the true base line equal to this drop s.

1f this dropa be taken as a measure of the distortion,l then, for short impulses,-it is obvious that the distortion is largely dependent upon the initial downward slope .of the line 64 which the record shows when a constant voltage or current is suddenly applied to the recording apparatus.

This downward slope, or decay, in turn, can be measured by drawing a tangent to the recorded curve. at the point where the voltage or current was applied. and extending the tangent line until it intersects the base line at ts, as shown in Fig. 6.

The line $4 applies to the decay before correction; the curve 65 (see particularly Fig. 10) is corrected. The time interval between t: and t1 will be dened here as the time constant To of the recording circuit. A slow decay and'small distortion corresponds to a large time constant. -while a rapid decay and greater distortion corresponds to a small time constant.

To keep the distortion small, the present practice is soto proportion the condensers and asociated circuits that this time constant shall be iarge. There are, however, practical diillculties involved in increasing this time constant. For example, as the time constant is increased, ease of operation is reduced, because of the length of time required for the apparatus to reach 01H- ing stability after adiustments are made. or switching operations performed.

According to a principle underlying this invention, a distortioncorrecting network is inserted into the recording circuits of such nature as to reduce the distortion. without changing the time constant, or decay, inherent in the recording circuit prior to the inclusion of the correcting network. In this way, the time required tor the apparatus to reach operating stability is not lengthened. The accuracy lof reproduction-of a slow-decay circuit is thus retained without sacricing the operatingl advantages o1 a rapiddecay circuit.

The characteristics oi this distortion-correcting network should be such that, it the rest o! the circuit be hypotheticaily assumed to produce no distortion. then the correcting network itself should introduce distortion inverse to that which it is intended to correct. The impulse ot the rectangular wave form of Fig. 5, tor example, would be distorted in a direction opposite to the direction indicated at Il in Fig. 8, as shown at 86 in Fig. '1.

I! the tendency of the corrective network .to record the constant voltage as a rising voltage balances the tendency of the rest ot the circuit to record the constant voltage as a decaying voltage, then the distortion is cancelled out.

It is not, of course. possible, in general, to find a physically realizable network which will cancel the distortion perfectly', but it is possible to approach such cancellation, and the approximation may be very close indeed for short-time intervals. It so happens that individual cardiac impulses are of such short duration as to make distortion correction for electrocardiography entirely practicable.

One such corrective network which has been foimd very satisfactory is shown in the output circuit of the last tube lli in Fig. 1, where a correcting resistance R1 and a correcting condenser Cr, connected in series, are shown connected in parallel to the resistance l2 and the galvanometer coil Il.

Let it be assumed that the internal plate Yimpedance of the output tube 30 ot the ampliiier is large compared to Ra. the'total resistance of the galvanometer plus any'resistance which' may be added in series thereto.

Let To represent the time constant of the amplifier with the resistance R1 and the condenser C1 lett out, and let T=R1Cn where Te is the time constant of the corrective network. It is approximately equal to the time' interval over which the corrective network `has some eect. For electrocardiography, and with an amplifier whose time constant To equals 1.5 seconds, it has been found satisfactory to set Te at .25 second.

For exact initial correction, that is, no initial decay, thefollowing equations. as will presently be showmmust be satisiied:

Having Ta. and knowing To, the time constant o! the circuit to be corrected, and Re.

' to a correcting network inserted in the plate circuit of some intermediate stage in a vacuum-tube amplier.

In thisl case. the resistance 18, for example, may be divided into two parts Ri and Rz, the former shunted by the condenser C1. The corrective network may be applied in the plate or the grid circuit or circuits oi any ampliiier stage or stages.

. n.lscioo' In Fig. 15, iorexamplgacorrectivenetwork com- It is prislng a resistance .Il and condenser il is .shuntedacrosstheplatecircuitoitheilrststase of the amplifier, a second corrective network comprising a resistance Il and condenser Il is shunted across the grid circuit of the third stage of the ampliiler. while a third corrective network comprising a resistance Il and condenser 12 is shunted across the output circuit of the ampliiler.

condenser 13 is shunted acrossa portion -oi the grid-cathode'impedance 2l of the second stage of the amplifier, a second corrective network 1'5 comprising the condenser 'I5 is shunted across a portion of the plate-cathode impedence 28 and 'I4 of the second stage of the amplier, while a third corrective network. comprising the resist ance 1l and condenser i3 is shunted across the4 im output circuit of the amplier.

in these cases, too, as in other cases within the scope of the inventiomthe characteristics of the corrective network are such that it reproduces a suddenly applied constant voltage as a rising 25 voltage whose initial upward slope. illustrated by the lline 66, exactly or approximately equals the initial downward slope, illustrated. by the line Bl, with which the circuit to be corrected reproduces the same .suddenly applied volte.

@il The initial slopes will 'beexactly 'equal il the circuit parameters are so adjusted t the sudden application of a constant voltage to the input of the corrected amplier, at the terminals 2, 4l, results in a'deection ofthe recording device 85 or indicator 40 in the output 'circuit of the output ampuner so of such nature that the initial decay is zero.

Figs. 3 to 7, as before stated, are matic. Figs. 8 and 9 illustrate actual cardiog'rnms.`

In Fig. 8, there is illustrated a cardiogram produced without the aid of the present invention. The time constant To-is 1.5 seconds. 'I'he triangular impulses. corresponding to the impulses` In Fig. 9. on the other hand, the corresponding/ l 'cardiogram, produced by the samev circuit after 50 correction in accordance with the present inven I tion, shows the theoretical perfection of Pig. I'

, 'Ihe decay curve of Fig. 10 clearly shows the initial attening of the curve after a sudden appllcation oi a constant voltage to the corrected 55 circuit.

advantage which this type of coring the sensitivity untilthe application of thispotential produces a standard deiiection of the re- 05 cordingbeamorshadowoionecentimeteronthe camerataceor'vlewinsscreen'. Iifvtheelectro cardiograph circuit contains series condensers. thenthe deflection decays immediately after the application o! the calibrating potential, so that the operatorl mt guess atthemaximmn amplitudeoithebesmdeilection. Withthiscorrec- A tive network, however. the initial decay is aero. sothatthebeamisconstantforashort timeinterval,thusallovwingalnoreaccurallsoiiv 75 servationoitheamvlitudcofthedencctinn.

In Fig. i6, a corrective network comprising ay now in lorderv to derive above equations.

1 Without the corrective network, the response to,

a suddenly applied constant voltage oi a circuit containing series-coupling condensers is some such decaying curve as-is shown in Fig. i1. Re-

gardless oi the exact shape oi thiscurve, it may be expanded into a series which will be valid for a short interval oi.' time after t=o. This series is The principle or this invention is to so design -the corrective network that the following relationship holds:

fas-

To c) ln Fig. 13, it is assumed that the source inter nal impedance. is high compared to Rs, so that it acts substantially as a constant-current source.

Then:

where is is a constant current o f amplitude I0 suddenly impressed at t=0.

The solution oi Equation 8 and any further dinernual equations w111 be carried out by the Laplacian Transform Method. (See Eschbach: Handbook of Engineering Fundamentals; page 2-108, and Campbell and Foster: -Fourier Integrals for Practical Applicatlon,.Bell Telephone f' System Monograph B-584.) v (e VYLetthe transforms o! the currents be, respec-1 tively,

' r Li,l=1, Li=1 Lua :In Transforming equation (8) toa function of the complex variable s, e

4 v' romeo-- 'mverungusnoobceinuomuonocume mman..

Comparing and (4), we have R. 1 -1 1 (n). =1R,[1+R (so) a'- RC RC compare 39) and 4). For exact initial correction L u-R C Y (40) 1 1 1 m ""i-f, 1 L

R c "T (41) i R.- R1 2 l s 3o Y -m T=R1C| f (42) R, Equations 41 and 42 determine the design of the v =T` (23) network with the exception of the choice of Rz. Y This naturally depends upon the tube used and 40 Lettini! the gain required of the stage.

later-.Te (24) The following analysis shows the actual respouse obtained when using the corrective neti-,zl (25) work of me. 1. when o. oonstant voltage ,n 1m- T R T pressed on the input of the amplmer at t=0, then 45 T. the voltage appearing at the grid of the output, R.=Rf5 (26) me wm be' e deoeymg vonage which, ror e s short-.time comparedtoTmcanbeexpreasedas RFA-'Rr 21) a-f? 5 y R1= T2-1] (as) mesme eurrenowmbegfmby;

c Y' L The derivation o! the design formulae for one Y 'i.=I. T (43) -o1thealternstivenetworkmmchasttntm Transforming, sstratedinrigl'ilinowbegivenlnoonnectlou I I 1 with m. 14. f 's 1 44) The internal impedance ol the tube is supposed tobehigheomparedtolksothatitmsyb Substitutlnsmu). considered to act as s eansnt-ment T:- I l D il* mt am @me RJ.= --I. R1+ (45) pressed at t=0. I S+ 1 C15 zr-n'. (2s) r1 Mas 1 5 dt==ili1 (30) Q l Transtormm (N) L Y I l V se 1 LR, p (an (Isa- T nc) 2G For t small compared to a Wee-o#wieQ1 'v 35 m1 no man decay, the term containing 1 must be aero, thatis,

a-p ieg-1)@ (53) 'no R,

eis-ii 5 1 1 R, ."n,ci i'1 m i 1 i R101 Roca ,(55)

Equation 55 1s identical wan 22 which waspreviously derived from the basis prlnclple oi the 50 inveztion 'that is expressed in Equations 3, s

^ and Equation 49 applies for a time interval short --compared to To, and so can be used to evaluate the actual decay of the deiiecticn tor s few so tenths 01's second after the constant voltage 1s applied. For a specific example take t .1` see- 0nd, `anti the circuit constants aiven above.

at end r 1mm. the lmnhasdecayedtoilpcrcentoiitsoriainalvalue, 76 orthedropislpercent.

for t small compared to tivenetworkthisdecaywouldhavebeendvm bstnefacwr C l El For 11. -nsrsna f-.1 ses.

the deflection would have dropped 'I percent as compared with 1.5 percent for the corrected circuit. l! no corrective network is used and similar results are desired, then the time constant of the amplifier would have to be 6.7 secs. as compared to 1.5 seconds for the corrected circuit.

In these calculations the important equations are numbered 3, 4, 5, 8, 17, 18, 20, 21, 23, 24, 28,

Equations 3, 4 and 5 express the principle oi the invention. y

Equation 8 is the dierential equation for the corrective network of Fig. 3'.

Equation 1'7 is. the solution of Equation 8. Equation 18 expresses thel exponential of Equation 20 ,applies the approximation o Equation 18 to Equation 17 so as to put Equation 20 in the form of Equation e.

Equation 21 brings out the similarity of Equom tions 20 and i.

5 to the network.

Equations 24 and 28 are the design formulae for the corrective network. of Fig. 3, resulting from Equation 22. A-

Equation is the dierential equation for the corrective network of Fig. t.

Equation is the solutionoi Equation '30. Equation 39 expresses Equation 35 in the required form of Equation 4 and is valid for a short time interval only.` Equations 40,41 and 42 are the design formulae for this network.

Equation 43 expresses the decaying current flowing in the plate circuit of the last tube of Fig. 1, when a constant voltage is impressed at the input of the amplier.

Equation 48 denes the factors a and in terms of which the' later work is written;

Equation 49 is the expression for the galvanometer current as a function oi time. It is valid for a time small compared to To.

EquationA 50 introduces the approximate expressions for the exponentials which are valid Equation 52 an approximation for the salvanometer current, valid for a times hort .-compared to Ts. This is derived from (49) by use 12000 10o0o"'1)]v fmust exist-among Equation 53 expresses the condition that `the galvanometer current be'independen't -oi time, that is, constant.

Equation 55 expresses the relationshipsl which the network parameters in ordertosatis'iyS). j

Equation gives the actual galvanometer current .l second-'after the application otithe 'constant voltagetothegivenamplierandcorrective network.

Further modications. will occur tol Equation 22 applies the principle oi Equation I skillediin the art,'and all considered to t fall within the spiritand scope oi the invention, f.

asA denned in the appended claims. Whatisc1ime dis: I

1. A method ot correcting the distortion that is produced in oscillographic-recording circuits by the presence therein of a series-coupling condenser or condenser-s, the introduction oi a corrective network which reproduces a sudadenly applied constant voltage as a rising voltage whose initial upward slope approximately or exactly equals the initial downward slope with whichthecircuittobecorrectedreproducesthe 10 same suddenly applied voltage.

2. In an oscillographic-recording circuit containing series coupling condensers and a recording device, a corrective network comprising a condenser -in series with a resistance shunted across .the recording device in sexies-parallel combination, the circuit parameters having values such that the sudden application o! a constant current, to the series-parallel combination results in an increasing deflection, the rate oi increase being equal to the rate of decay of the deflection produced when a. constant voltage is applied to the input of the recording circuit without the inclusion of the corrective network.

3. In a recording system containing an amplifier comprising one or more resistance-capacitance-coupled ampliiier stages each comprising a vacuum tube having a cathode, a grid and a plate. an input circuit containing the cathode andthe grid, and an'output circuit containing the cathode and theplate, and a recording device in the output of the last stage, a corrective network comprising a condenser in series with a resistance shunted across one of the said circuits' of onev or more of the stages for correcting the distortion that is produced by the presence of the coupling condenser or condensers oi the resistance-capacitance-coupled ampliner stages, the circuit parameters having values such that the sudden application of a constant voltage to v 40v the input of the amplifier results in a deiiection of the said recording device whose initial decay is zero. A

4 Inanelectricsystemcomprisingacutput circuit having aresistanceRz,ted from a source theinternalresistanceofwhichislarge compared with Rz, a. corrective network comp aistanceR1andacondenser1 M I Ria :i l)

whereTsistheiimeconstantot-thecorrective netvrrorlc,a'ndl'sisthetiincconstani'foithiesys-l tembefore correction.

5. Inaresistan'ampliiiertheoutputcircuitoitheontputampliner wnm'riinhsamemntofmemwme network. and Tais the time-conatant'oti'ho ampliner. s 6. Inaneloctricsyltem.adronithavinga arcaico corrective network comprising resistances Ri and Rs and a condenser Cx in parallel to the resistance R1, fed from a source the internal resistance of which is large compared with Ra, and having the valuesl 5 Tg: Rlcl i where Ts ia the time constant of the corrective 10 sistances R1 and Rz and a condenser Ci in par- 15 allel to the resistance Ri, fed from a tube the internal plate resistance of which is large compared with Rz, and having the values L L 20 mc, T.

where Ts is the time constant of the corrective network. and To is the time constant of the 25 amplier before correction.

8. An electrocardiograph having, in combination, an amplier comprising a plurality of resistance-capacitance-coupled amplifier stag. each comprising a vacuum tube having a cathode, 30 a grid and a plate, an input circuit containing the cathode and the grid and an output circuit containing the cathode and the plate, the ampliiler having one or more series condensers, a piurality of electrodes connected with the input cr- 35 cuit of the first stage and through the medium o! which the electrical impulses associated with cardiac activity may be impressed upon the said input circuit of the said rst stage. an indicating device in the output circuit of the last stage and 40 l to which the said electrical impulses are adapted to be transmitted after ampliiication by the amplier, and a corrective network in one or more of the stages for correcting the distortion that isproduced bythepresenceoithesaidseries condenser or condensers, the circuit parameters having values such that the sudden application ci a constant voltage to the input of the system wouldbereproduced.iftheaeriescondensersare' .innnitelyiargaasarisincvoitagewhole Il initial upward slope approximately or exactly equalstheinitialdownwardsiopewithwhichthe uncorrected electrocardlograph reproduces the same suddenly applied voltage.

9.Inanelectriccircuitcontaining cneormore Il series-colmiingcondensersandaloadacorrective network comprising a in series with a resistance shunted across the load in seriesparallel comhinatiomthecircuit parameters havingvaluessuohthatthesuddenapplicationoi U a constant current to the series-parallel combinationresultsinanincreasingvoltageacroastho 'load,thorateodincreasebeingequaltotherate ofdecayoithevoltageacrstheloadprcded whenaconstantvoltageisappliedtotheinput otthesaidelectriccircuitwithoutthoincluaion `ot iecorrectivc network.

lmanelectrocardiographlnvingincomhination,anampliiiercomprisingapluralityofre tance-coupled ampliiior M70 eachcomprisingavacimmtubehavingacathode. agridandaplataaninputcircnitcontainingtho cathodeandthegridandanmxtpntcircuitcontainingtheoathodeandtheplatatheampliiicr h'lnworfmoroserlsandmc Il or more resistors between adjacently disposed stages, a plurality of electrodes connected with the input circuit of the rst stage and through the medium of which the electrical impulses associated with cardiac activity may be impressed upon the said input circuit of the said first stage,

` an indicating device in the output circuit of the last stage and to which the said electrical iml pulses are adapted to be transmitted after amplicationby the ampliier and a corrective network in one or more of the stages in series with one or more of the said resistors for correcting the distortion that is producedl by the presence of the said series condenser or condensers, the network comprising a resistor in parallel with a condenser and having circuit parameters of value such that the sudden application o! a constant voltage to the input of the amplitler results in a deection of the said indicating device whose 20 initial decay is zero.

. pliner comprising one or more resistance-capacitance-coupled amplier stages each comprising a vacuum tube having a cathode, a grid and a plate, a grid-cathode impedance for each stage, a plate-cathode impedance for each stage, an

input circuitcontaining the cathode, the grid and the corresponding grid-cathode impedance, an output circuit containing the cathode, the plate, and the corresponding plate-cathode impedance, and a recordingdevice in the output ofthe last stage, a corrective network comprising a condenser shunted across a portion o! one of the impedances oi oneor more of the stages for correcting the distortion introduced by the coupling condenser or condensers of the resistance-capacitance-coupled amplifier stages, the circuitparameters having values such that the sudden application of a constant voltage to the input of the amplier results in a deflection of the said 'recording device whose initial decay ls zero.

. ARTHUR MILLER. 

